The present invention relates generally to optical transducers such as laser diodes and solid state laser/external modulator combination, and is particularly concerned with a circuit or device for reducing or cancelling laser noise or nonlinearity components in the external modulator output signal.
The use of fiber optics for remote, high speed transfer of analog data having different frequencies has been limited up to now by the limited dynamic range and fidelity of currently available optical modulators or laser sources. The laser diode sources which can be directly modulated, have relatively high noise levels which can mask weak signals. The most obvious solution to this problem would be to develop a laser source which generates less inherent relative intensity noise (RIN). Laser manufacturers are aware of this and are currently investing considerable research efforts and expense to improve RIN. However, the best currently available laser sources produce equivalent noise figures of around 50 to 60 dB, which are not normally acceptable for remote sensor processing. Reduction of the inherent noise level in the laser source itself is likely to considerably increase the cost of the laser source. Laser sources which exhibit much better noise levels require that modulation be accomplished externally. The external modulator uses a continuous-wave low noise solid state laser as the optical source. Thus, the RIN noise does not limit the transducer's dynamic range. Rather the nonlinearity of the device limits the dynamic range. The improved laser noise level is obtained at the expense of distortion produced by the modulator. The dynamic range is also limited but by reason of distortion created by the modulator rather than due to the noise floor.